The invention pertains to the field of Digital-to-Analog (D/A) converters.
The direct conversion of multiple-bit digital signals to bidirectional analog currents has become especially useful only very recently. The Compact Disk (CD) audio recording system, which is gaining in popularity, uses a digital recording technique, rather than the analog technique used on conventional records and tapes. The audio signal representing the music is sampled 40,000 to 80,000 times per second. The signal voltage at that instant of time is expressed as a 16 bit digital number, and is recorded optically on the disk. This has major advantages, since the digital number is not subject to surface noise, hiss, distortion, or the other recording problems found in analog recording, because each digital number remains the same, even if there may be distortions in the recording of each bit.
CD players using the current technology read the disk using a laser, and then immediately reconvert the digital number to a small analog voltage using a conventional D/A converter. The analog voltage is then amplified conventionally in a conventional amplifier, and fed as an analog voltage to drive the loudspeaker. Unfortunately, this means that the advantages of digital recording are lost the instant the signal leaves the disk, as it were. Each of the succeeding analog amplification stages after the D/A converter introduces additional distortion and noise. On the other hand, the output signal from the D/A converter is too small to drive a loudspeaker. If one could directly use the digital signal from the disk to drive the loudspeaker, amplifying only the digital signal and converting to analog only in the positioning of the speaker cone, then the distortions and noise involved would be eliminated.
The ability to steer current bidirectionally and accurately based upon a digital source allows the signal to accurately position a loudspeaker cone. With unidirectional current, the cone can only be pulled in one direction, with a spring or cone suspension being responsible for returning the cone to a neutral position. Combining the bidirectional current steering with direct digital amplification leads to reproduction of the digitally encoded sound which is especially free from distortion and noise.
Existing D/A converters do not have the capacity to force bidirectional currents through a load. Most existing (see FIG. 1) D/A converters utilize binary weighted current sinks and active devices for switch pairs. These switch pairs direct the current sinks to one of two outputs. One terminal of the load is grounded and passes this current, which ranges from zero to the full scale supply current (Ifs).
If bidirectional current is needed (FIG. 2), an external current source is sometimes connected to supply a current of 1/2Ifs, allowing currents of -Ifs to zero to +Ifs to be output by the D/A converter. By the standard formula P=I.sup.2 .times.R, the load power in such a case is (1/2Ifs).sup.2 times the load resistance (R1), or one-quarter of the maximum power available (Po) (where Po=(Ifs).sup.2 .times.R). This is inefficient. Further, mismatches in the current source as opposed to the total sink current provided by the D/A converter introduces a source of inaccuracy.
Another approach to bidirectional current from a D/A converter is a bridge arrangement (FIG. 3), which uses an extra sign bit to direct the D/A converter output in alternate directions through a load. To change the output continuously from zero to +Ifs to zero to -Ifs (to form a sine wave, for example) one would need to key the D/A converter in an unusual code progression not usually found in digital systems, or, to put it another way, a standard binary progression from zero to the maximum would not result in a smooth progression in output. In the application of music reproduction from CD encoded sources, this is obviously of crucial importance.
U.S. Pat. No. 4,408,094, issued to Oura in 1983, shows an output circuit for digitally-encoded audio signals using a D/A converter driving a loudspeaker with a bi-directional signal. Oura's D/A converter, however, does not provide bi-directional output. Rather, Oura converts the digital signal to a uni-directional analog current signal in a D/A converter, then feeds the output to a bi-directional output stage. The output stage is a bridge circuit using 4 transistors driven by a voltage source (not matched current sources), in which the direction of the analog output current from the D/A converter is switched in accordance to a direction bit. The single voltage reference may be changed to act as a volume control.
Von Sichart, U.S. Pat. No. 4,415,883 [1983], shows the use of R-2R networks in D/A converters. The D/A converter provides unidirectional current, as do other D/A converter arrangements in Craven (U.S. Pat. No. 4,309,693 [1982]) and Smith (U.S. Pat. No. 4,383,248 [1983]).
Siligoni (U.S. Pat. No. 4,502,016 [Feb. 26, 1985]) shows a bridge circuit used as a final stage in an audio amplifier. The circuit is purely analog.
It is thus an object of the invention to provide a D/A converter which can be used to directly translate digital data into accurate analog current without requiring conversion of the digital code from standard coding.
It is a further object of the invention to provide a D/A converter which can efficiently produce accurate bidirectional current through a load in response to a multiple-bit digital input.
It is a further object of the invention to provide a D/A converter which can accurately and efficiently provide large enough bidirectional currents to drive a loudspeaker or similar load.